Synovial sarcoma arises in the para-articular tissues, and it can also occur in various unexpected sites. We report a rare case of primary monophasic
synovial sarcoma (MSS) arising in the mesentery. A 59-year-old man presented with a palpable abdominal mass. On microscopic examination, the entire
tumor comprised a dense proliferation of the spindle cells without epithelial components. The
tumor cells were positive for
transducin-like enhancer of split 1, bcl-2,
epithelial membrane antigen and CD99 but negative for CD34, CD117, alpha-smooth muscle actin,
cytokeratin, and
calretinin on immunohistochemistry. The
reverse transcriptase-polymerase chain reaction revealed a single 151-bp fragment representing the
SYT-SSX2 fusion transcript. Because mesenteric MSS is extremely rare and many cases display histologic findings that overlap with those of more frequently involved
tumors such as
hemangiopericytoma and
gastrointestinal stromal tumor, there is a chance of making an incorrect diagnosis that can result in an inappropriate treatment.